The present invention relates to an optical fiber assembly and, more particularly, to an optical fiber assembly for transmitting a laser beam of high energy.
An optical fiber is used to transmit a laser beam in an optical communication system, an optical measurement system, a medical application system such as a laser knife in medical treatment and a machining system in industrial processes such as a marking, machining, cutting and engraving system. This optical fiber includes a core and a cladding for covering the core. The outer periphery of this cladding is covered with a protecting film, namely a primary coat of silicon and a jacket of polyamide.
In general, when a laser beam is transmitted through such an optical fiber, the laser beam is converged into a core in the range of the accepting angle of the optical fiber which is defined by the numerical aperture (NA) of the fiber and is transmitted through the core from the incident end to the emitting end. However, the directivity and the beam divergence of the laser beam is varied when the laser output power is changed, thereby causing the beam to fluctuate. So that, when the beam is focussed on the end face of the optical fiber, the beam spot is not only formed on the core section on the end face of the optical fiber but is also formed on the cladding section due to the fluctuation of the beam. In other words, the main component of the beam is transmitted through the core, but the other component of the beam incident on the cladding is propagated through the cladding.
When a higher energy than 20 or 30 watts; e.g., several kilowatts is transmitted by the optical fiber as in a carbon dioxide gas laser or a YAG laser energy transmission, the protecting film on the outer periphery of the cladding at the beam incident face of the optical fiber is often damaged by the temperature rise caused by laser beam leakage from the cladding to the film or by the beam being directly incident on the film. In these cases, the optical fiber may also be dropped from the member for holding the fiber because of film softening caused by this temperature rise. Similar problems occurs at the light emitting end when a portion of the laser beam is reflected from the reflecting material located near the emitting end to the above-mentioned emitting end. In this case, the same type of problem exists at the emitting end as discussed above with respect to the light incident end.
Heretofore, in order to prevent the protecting film of the optical fiber from being damaged, the beam incident on the optical fiber has been limited in power to about 20-30 W. An optical fiber in which the protecting film is not readily thermally damaged is strongly desired for a high energy laser beam transmission.